The low-resolution tachometers currently available for use on Industry Standard Frame Size AC and DC motors generally consist of a magnet biased sensor magnetically coupled to a metal gear-toothed wheel (the wheel being fastened to rotate with the DC motor output shaft). Relatively low pulse counts of 60, 120 and 240 pulses per revolution ("PPR") are typically obtained by providing an appropriate number of teeth on the gear-toothed wheel. Such gear wheel arrangements are useful in certain low-resolution applications, but have several disadvantages in other applications.
One major disadvantage of such standard tachometers is that different wheels must be provided for different pulse counts. This means that the manufacturer must make (and inventory) several different types of gear-like wheels having different numbers of teeth, i.e., 60, 120 or 240. This significantly increases design and manufacturing costs.
In addition, the resolution of prior art gear tooth (and optical) arrangements (whether of standard or slim profile design) is currently limited to about 240 PPR. For the gear-tooth type tachometer, this is because the upper limit on pulse count is a function of how many teeth can be placed accurately on the wheel and on the ability of the sensor to differentiate between the teeth. Mechanical durability and machining tolerances also limit the number of teeth that can be placed on the wheel. Moreover, speed variations, the size of the magnetic sensor, and other factors require a minimum spacing between teeth to ensure that each tooth is detected.
An improved magneto-resistive tachometer, which overcomes many of the disadvantages of known devices, is disclosed in commonly owned U.S. Pat. No. 5,293,125 , the entirety of which is incorporated herein by reference.
Briefly, that patent application discloses a magneto-resistive sensor arrangement utilizing a non-magnetically biased magneto-resistive sensor element; a magnetized drum; and associated integrated circuit-based electronics. More specifically, the sensor arrangement includes an enclosure, a magneto-resistive probe, a drum secured to a hub, and an electronics module. The generally cylindrical enclosure is adapted for bolting onto, for example, the standard NEMA 4.5 inch (or 8.5 inch) C-face end plate (also known as a NEMA 56C face) of an AC or DC electric motor. In one exemplary embodiment, the hub is fastened to the end face of the motor shaft and a magneto-resistive drum is fastened to the hub. The drum includes two magnetized tracks: an incremental magnetic (INC) track and an index pulse (Z) track. Such magnetized tracks are formed in a conventional manner by magnetizing the periphery of the drum. The Z track encodes one pattern (pulse) per revolution, while the INC track encodes, in the preferred embodiment, 480 patterns (pulses) per revolution. The INC track may provide for up to 1024 patterns (pulses) per revolution.
In known slim profile tachometer designs, whether of the gear tooth or optical type, resolution, as already noted above, is currently limited to about 240 pulses per revolution. Slim profile designs are nevertheless desirable where components are located in series, along the output shaft of the motor. For example, a shaft brake may be mounted on the shaft, adjacent the tachometer and on the side thereof remote from the motor casing.
It is the object of this invention to utilize the magneto-resistive technology and interchangeability features of the tachometer disclosed in the '016 patent application, but in a slim profile tachometer design. In an exemplary embodiment, a slim profile enclosure or housing in the form of a thick rectangular plate or block is provided with a large circular aperture formed therein. The block or plate is otherwise substantially smooth on its exterior surfaces and is adapted to be secured by bolts or other suitable fasteners to a motor face having a standard NEMA rabbit, i.e., the NEMA 56C face. The enclosure central aperture is counterbored to provide a pair of radial steps or shoulders, such that three distinct diametrical surfaces are provided, the intermediate one of which is a critical diameter which exposes a sensor element to the magnetically encoded peripheral surface of an associated drum or adaptor fixed to the motor shaft. In other words, the encoded drum is slideably located on the motor shaft and moved into position within the enclosure such that the encoded, outer peripheral surface of the drum is radially adjacent the intermediate diameter of the enclosure, and spaced from the sensor element by a critical radial gap, as explained further herein.
In a typical upright position, a slot is formed in the enclosure at the 12 o'clock position, communicating the interior of the central aperture with an enlarged recess located above the central aperture and utilized to house the sensor electronics package. Within this slot, a sensor element is fixed at the above mentioned radial gap relative to the peripheral surface of the drum. It will be appreciated that the innermost diametrical surface within the central aperture and its associated radial shoulder, prevent the drum from sliding axially beyond the enclosure in a rearward direction toward the motor face. It will also be appreciated that the drum is easily removed from the front of the enclosure by sliding in an opposite or axially forward direction, i.e., away from the motor. It is also a feature of the invention to provide a removable cover for the front of the enclosure, to protect the drum, sensor element and associated electronics.
Another feature of the invention relates to the use of interchangeable drums, each provided with magnetic INC and Z tracks enabling different PPR outputs from 60 to 1024.
Another feature of the invention relates to the manner in which the drums are secured to the motor shaft. In the preferred embodiment, each drum includes a central hub connected to the peripheral portion thereof by three radial spokes. One of these spokes is slotted and incorporates a clamping screw so that upon tightening of the screw, the spoke compresses, thereby also causing the hub to be tightened about the motor shaft. In the preferred embodiment, the slotted spoke halves are also provided with oppositely facing axial openings for locating the clamping screw within the spoke. In this way, the drum can be cast, thereby eliminating the need for machining or drilling holes to accommodate the clamping screw.
It is another feature of the invention to provide a hardened washer on the clamping screw and within the slotted spoke to prevent overtightening of the clamping screw and thus preclude an out-of-round condition for the peripheral surface of the drum which, in turn, would negatively impact the performance of the tachometer.
In one aspect, therefore, the invention relates to a tachometer comprising:
a) a slim profile enclosure having a thickness of about 3/4" and formed with a central through opening therein, and a plurality of bolt holes sized and located for alignment with bolt holes on a standard NEMA C-type motor end face; the enclosure having a recess formed adjacent the central opening and adapted to receive a sensor electronics package, and a radial slot between the central through opening and the recess for receiving a sensing element; and
b) a first selected drum having a peripheral surface with encoded sensor patterns thereon adapted to be sensed by the sensing element, the drum having a center hub and a peripheral sensor surface connected to the center hub by a plurality of spokes, the center hub adapted for clamping engagement with the output shaft of the motor within the center opening of the enclosure and in radially adjacent position relative to the sensing element, at least one of the spokes provided with shaft clamping means for securing the drum to the output shaft of the motor.
In another aspect, the invention relates to a magneto-resistive tachometer comprising:
a) a thin profile enclosure formed with a central through opening therein, and a plurality of bolt holes sized and located for alignment with bolt holes on a standard NEMA C-type motor end face; the enclosure having a recess formed adjacent the central opening housing a sensor electronics package, and a radial slot between the central through opening and the recess for receiving a sensing element; and
b) a plurality of interchangeable sensor drums for selective use with said enclosure, each drum having a peripheral surface with encoded sensing patterns thereon such that each drum provides a different number of pulses per revolution; each drum having a center hub connected to the peripheral surface and means for locking said hub to the output shaft of the motor.
In still another aspect, the invention relates to a tachometer enclosure comprising a block having a thickness of about 3/4" and formed with a central through opening therein, and a plurality of bolt holes sized and located for alignment with bolt holes on a standard NEMA C-type motor end face; the enclosure having a recess formed adjacent the central through opening and adapted to receive a sensor electronics package and a radial slot between the central through opening and the recess for receiving a sensing element.
The slim profile, variable pulse tachometer of this invention provides many advantages to both the manufacturer and the customer. Some of the advantages accruing to the manufacturer follow.
1. A small number of parts are required for the construction of the tachometer.
2. Low tolerance (and hence low cost) machining for much of the enclosure is permitted by otherwise precise location of the sensing element.
3. The single style enclosure has many uses, and this allows the critical parts to be produced in high volume rather than a multitude of low volume parts.
4. Junction boxes and other coupling accessories can be attached if needed, thus increasing the versatility of the basic design.
5. Drum assemblies are interchangeable with the enclosures, so that components need not be matched. In practice, an in-house supply of enclosures and a variety of drums will be kept in stock for quick customer response.
6. The drum may be made from a plastic material which is resistant to damage. Other types of magnetic pulse wheels use a thin deposited magnetic layer on an aluminum drum, and this magnetic layer can be scratched if abused.
7. Pigtail cable, standard on all units, allows many coupling options with one standard cable.
Some of the advantages accruing to the customer are listed below.
1. This invention provides the highest pulse per revolution (PPR) output available (1024 PPR), using magneto-restrictive or Hall Effect technology, versus 120 or 240 PPR offered by others.
2. Desired pulse counts can easily be achieved simply by changing the drum size, the sensor and/or pulse division.
3. The tachometer of this invention is easy to assemble and install since it consists of just two mechanical parts, plus whatever is needed for electrical connection.
4. The device is self-aligning, so that no customer adjustment is required.
5. The tachometer of this invention is characterized by an absence of bearings or couplings. Being able to mount directly on the motor without bearings is a great advantage over many of the stand alone units available today which have bearings and require shaft coupling to the motor.
6. The tachometer enclosure is only 3/4" thick, thus allowing applications where only limited space is available.
7. The customer can add a ring to the outer or forward side of the enclosure to make a double C-face configuration. This allows the unit to be attached to a motor face on one side, and to another accessory on the other side of the enclosure, e.g., gear boxes, brakes or other accessories.
8. Similar tachometers with different PPR drums can be stacked together on the motor shaft to provide multiple outputs.
9. The tachometer can be assembled into units with bearings to make stand alone units.
10. The tachometer can be fitted onto either the drive end or accessory end of a motor.
11. The pulse wheel or drum is also relatively thin to allow it to fit within the slim profile (3/4") enclosure.
12. The interchangeable drums can be designed to fit over shafts up to 2.875" in diameter.
13. The enclosure may be made of aluminum which lowers cost, but is rugged enough to withstand mounting on motors and other accessories. A hard coat anodization may be applied to the aluminum enclosure.
14. Stainless steel versions will be provided for both the code wheel or drum and the enclosure where desired.
15. The electronics package is fully encapsulated for protection against humidity and harsh chemicals.
16. The quadrature angle is set within the sensor itself. Other known styles have two independent sensors that must be adjusted to gain quadrature.
17. The unit is essentially non-repairable, and the drum and/or enclosure are easily replaced if defective.
18. The drum assemblies are interchangeable with the enclosures. Thus, they do not have to be kept matched to each other.
Additional objects and advantages of the present invention will become apparent from the detailed description below.